Inspection of the electric circuits formed for semiconductor substrates, liquid-crystal displaying devices, and other devices is usually conducted by using an inspecting apparatus provided with a multitude of contact probes. The surface of a circuit to be measured (hereinafter referred to as a subject surface) is usually covered by an insulating layer, such as a naturally formed oxide layer or resist residues. To carry out the inspection, it is necessary to break the insulating layer to secure reliable electrical contact with the electrode of the circuit underneath the insulating layer. Two methods have been employed to break the insulating layer. One is to scrub along the subject surface to remove the insulating layer so that the electrical contact with the electrode underneath can be secured. The other is to press a sharp edge against the insulating layer to penetrate through it.
In this specification, the term “scrub” is used to mean to scrub a subject surface by using a sharp edge. Researchers and engineers have proposed a process known as the lithographie galvanoformung abformung (LIGA) process for forming a contact probe that performs the foregoing “scrubbing.” According to this process, a contact probe is formed by lithography and plating using a mask having a specified pattern as explained in the published Japanese patent application Tokukai 2001-343397, for example.
Examples of the conventional shape of contact probes formed by the LIGA process are shown in FIGS. 12 and 13. These contact probes comprise a tip portion 1 for making contact with a subject surface 20, a spring portion 2 having a bent portion, and a supporting portion 3 for supporting the contact probe attached to an inspecting apparatus. The shape of the spring portion 2 is not limited to the unidirectionally bending shape as shown in FIGS. 12 and 13; an S shape or a waving shape also can be used. As shown in FIG. 14, a contacting flat face 10 is provided at the lowermost part of the tip portion 1. An oblique face 15 is provided at each side of the contacting flat face 10. When the supporting portion 3 of the contact probe shown in FIG. 12 is fixed to an inspecting apparatus and when the contact probe is pressed perpendicularly against the subject surface 20, the contacting flat face 10 of the tip portion 1 makes area contact with the subject surface 20, and the spring portion 2 deforms elastically in a direction shown by an arrow 31. During this elastic deformation, the posture of the tip portion 1 is constrained by the force that presses it against the subject surface 20. Consequently, the tip portion 1 moves in a direction shown by an arrow 32 nearly maintaining the posture of the area contact with the subject surface 20. Thus, the “scrubbing” is performed.
The performance of the contact probe shown in FIG. 13 is similarly explained below. When the supporting portion 3 of the contact probe is fixed to an inspecting apparatus and when the contact probe is pressed perpendicularly against the subject surface 20, the contacting flat face 10 of the tip portion 1 makes area contact with the subject surface 20, and the spring portion 2 deforms elastically in a direction shown by an arrow 33. During this elastic deformation, the posture of the tip portion 1 is constrained by the force that presses it against the subject surface 20. Consequently, the tip portion 1 moves in a direction shown by an arrow 34 nearly maintaining the posture of the area contact with the subject surface 20. Thus, the “scrubbing” is performed.
The moving direction of the tip portion 1 pressed against the subject surface 20 is determined by the shape and the relative position of the tip portion 1, the spring portion 2, and supporting portion 3 when the contact probe is formed as a unitary structure with the same material. When different materials are used for individual portions, the types of the materials are also a factor to determine the direction.
A series of the operation from the start of the contact of the tip portion 1 with the subject surface 20 to the end of the contact is explained in detail below by referring to FIGS. 15 to 17. In the case of the example shown in FIG. 15, the subject to be measured is a substrate 21 provided with an aluminum electrode 22 on the surface. The surface of the aluminum electrode 22 is the subject surface 20. The surface of the aluminum electrode 22 is covered with a naturally formed oxide layer 25. As shown in FIG. 15, the tip portion 1 of the contact probe descends from immediately above in order for the contacting flat face 10 to make contact with the subject surface 20. As shown in FIG. 16, the elastic deformation of the spring portion 2 (not shown in FIG. 16) moves the tip portion 1. In the case of the example shown in FIG. 16, the tip portion 1 is assumed to move to the right. In this case, because the tip portion 1 is pressed against the subject surface 20, it moves to the right maintaining the posture of the contact between the contacting flat face 10 and the subject surface 20. Thus, the “scrubbing” is performed. During this operation, the tip portion 1 scrubs away the oxide layer 25, forming scratches 24. Under this condition, the tip portion 1 can secure the electrical contact with the aluminum electrode 22 previously covered by the oxide layer 25, enabling the measurement through the contact probe.
After the measurement, the contact probe ascends. However, the tip portion 1 does not ascend directly from the position shown in FIG. 16. As shown in FIG. 17, as the spring portion 2 decreases its elastic deformation, the tip portion 1 moves to the left while continuing to scrub the surface and then moves upward.
As shown in FIGS. 16 and 17, the scrubbing allows shavings 23 of the aluminum electrode 22 and the oxide layer 25 to adhere to the tip portion 1. Every measurement with the contact probe requires the operations shown in FIGS. 15 to 17. The shavings 23 adhere to the tip portion 1 not only when the scrubbing is performed by pressing the contact probe to the subject surface 20 as shown in FIG. 16 but also when the scrubbing is performed during the dissociation of the contact probe from the subject surface 20 as shown in FIG. 17. The latter scrubbing does not contribute to the measurement, although it is inevitable. The adhering shavings 23 decrease the quality of the electrical contact of the contact probe in the next measurement. Consequently, to maintain good electrical contact to a certain extent, it is necessary to clean the tip portion 1 after a certain number of measurements are carried out, for example, 1,000 times. The cleaning requires discontinuation of the measurement, thereby decreasing the productivity.
Excessive adherence of the shavings 23 diminishes the measurement accuracy, and a good product may be judged as unsatisfactory. This misjudgment causes an otherwise unnecessary reduction of yield.
Furthermore, as described above, the tip portion 1 produces the scratches 24 on the subject surface 20 not only when the scrubbing is performed by pressing the contact probe against the subject surface 20 for securing the electrical contact as shown in FIG. 16 but also when the scrubbing is performed during the dissociation of the contact probe from the subject surface 20 after the measurement as shown in FIG. 17. Consequently, the scratches 24 on the surface of the aluminum electrode 22 grow in excess of the extent necessary to break the insulating layer. The excessive scratches may cause unsatisfactory bonding between the aluminum electrode 22 and a gold wire in the subsequent ultra-sonic bonding process, because the alloying process between the aluminum electrode 22 and the gold wire does not proceed properly.
In addition, the subject surface is not necessarily a flat surface. It may be a curved surface, such as the surface of a ball. For example, a ball grid array (BGA) package 70 shown in FIG. 28 requires measurement by making contact with solder balls 72 arranged on the undersurface of a BGA substrate 71. Several contact probes have been proposed for the inspection through the foregoing solder balls 72. These contact probes and problems caused by them are explained below.
A first prior art for a ball-shaped subject surface uses a contact probe 100 shown in FIG. 29, which is one of the POGO® pin types. The contact probe 100 comprises a tip portion 121 for making contact with a subject surface and a spring portion 122 for connecting the tip portion 121 to a cylindrical supporting portion 123. The contact probe 100 is produced by machining. The tip portion 121 and the supporting portion 123 basically have a cylindrical shape, and the upper end of the tip portion 121 has a conical shape. The spring portion 122 is made of a coil spring. As shown by an arrow in FIG. 29, the contact probe 100 placed immediately under a solder ball 72 moves upward and causes the conical end of the tip portion 121 to penetrate through the insulating layer formed on the surface of the solder ball 72 so that electrical continuity with the solder ball 72 can be secured.
However, as shown in FIG. 30, the contact probe 100 leaves a dent 53 on the solder ball 72 after the measurement. As shown in FIG. 31, when the solder ball 72 having the dent 53 is used for soldering with a pad electrode 73 on a circuit substrate 74, the dent 53 forms an enclosed space surrounded by the solder ball 72 and the pad electrode 73. When the assembly is heated for soldering under this condition, the air in the enclosed dent 53 expands and may burst the solder ball 72. This phenomenon known as the “popcorn phenomenon” causes unsatisfactory connection, which is a serious problem.
A second prior art for a ball-shaped subject surface has proposed a contact probe 101 shown in FIG. 32. The contact probe 101 comprises a pair of arms 114 that can open and close like a pair of tweezers. Each of the arms 114 is provided at the tip portion with a claw 112 that faces the other claw 112. As shown by an arrow in FIG. 32, the contact probe 101 rises from below. As shown in FIG. 33, the pair of arms 114 move in a direction toward the closed position so that the claws 112 can engage with the solder ball 72 from the side. As a result, the insulating layer on the surface of the solder ball 72 is broken, and the electrical continuity between the contact probe 101 and the solder ball 72 can be secured.
However, the contact probe 101 has a drawback. It is difficult to adjust the closing movement of the arms 114 against the diameter of the solder ball 72. If the degree of the closing of the arms 114 is insufficient, the claws 112 cannot penetrate sufficiently. Conversely, if the degree of the closing of the arms 114 is excessive, the claws 112 penetrate excessively, damaging the solder ball 72 or making themselves locked. If the locking occurs, the claws 112 cannot be separated from the solder ball 72. Consequently, when the contact probe 101 descends, the claws 112 tear off the solder ball 72 from the BGA substrate 71, creating a problem. The contact probe 101 has another drawback in that it requires a complex mechanism for opening and closing the arms 114.
A third prior art for a ball-shaped subject surface has proposed a contact probe 102 shown in FIG. 34. The contact probe 102 has a cylindrical tip portion whose upper end forms a sharp edge 115. When the contact probe 102 is used, as shown by an arrow in FIG. 34, the cylindrical tip portion rises toward the solder ball 72. As a result, as shown in FIG. 35, the edge 115 penetrates through the insulating layer on the surface of the solder ball 72, securing the electrical continuity between the contact probe 102 and the solder ball 72.
However, it is difficult to produce with high precision the cylindrical tip portion of the contact probe 102. If the edge 115 has a diameter larger than that of the solder ball 72 to a certain extent, the edge 115 may push the solder ball 72 into the cylindrical tip portion of the contact probe 102 without penetrating into the solder ball 72 as shown in FIG. 36. In this case, an accurate measurement cannot be conducted because the insulating layer on the surface of the solder ball 72 is not broken. If this phenomenon occurs, when the contact probe 102 descends, it may tear off the solder ball 72 from the BGA substrate 71.